fix: Error codes needs to be documented in Swagger

[William-Hill]

Fixes #576

Agent Summary

0/v1/connectors

http://localhost:8080/v1/pipelines/running-pipeline-id/start
Scraping http://localhost:8080/v1/pipelines/running-pipeline-id/start...
HTTP error occurred: [Errno 61] Connection refused
Failed to retrieve content from
http://localhost:8080/v1/pipelines/running-pipeline-id/start
Initial repo scan can be slow in larger repos, but only happens once.
Looking at the GitHub issue content, I can see this is about implementing better
error handling for client-side validation failures in the Conduit API. The issue
requests that validation errors should return HTTP 400 instead of HTTP 500, and
be properly documented in the OpenAPI specification.

Let me analyze the provided files and implement the requested changes:

proto/api/v1/api.proto

@@ -459,11 +459,23 @@
           schema: {
             json_schema: {ref: ".google.rpc.Status"}
           }
           examples: {
             key: "application/json"
-            value: '{ "code": 9, "message": "failed precondition error", 
"details": [] }'
+            value: '{ "code": 3, "message": "failed to start pipeline: can\'t 
build pipeline without at least one source and one destination connector", 
"details": [] }'
+          }
+        }
+      }
+      responses: {
+        key: "409"
+        value: {
+          schema: {
+            json_schema: {ref: ".google.rpc.Status"}
+          }
+          examples: {
+            key: "application/json"
+            value: '{ "code": 6, "message": "pipeline is already running", 
"details": [] }'
           }
         }
       }
     };
   }
@@ -552,5 +564,6 @@
 }
 message GetPipelineResponse {
   Pipeline pipeline = 1;
 }
 
+ 556 / 1085 lines [███████████████░░░░░░░░░░░░░░░]  51%

Model openrouter/qwen/qwen3-coder has hit a token limit!
Token counts below are approximate.

Input tokens: ~54,772 of 262,100
Output tokens: ~4,056 of 262,100
Total tokens: ~58,828 of 262,100

https://aider.chat/docs/troubleshooting/token-limits.html

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Generated by conduit-agent-experiment (archivist: Gemini Flash, implementer: openrouter/qwen/qwen3-coder, 1 iterations).